1. Field of the Invention
The invention relates to a ceramic composition, more particularly, to a multilayer ceramic composition for the performance of an electronic microwave system.
2. Description of the Related Art
Interconnect circuit boards are necessary for modern electronic products to meet the requirement of being light, thin and small. The interconnect circuit boards are either electronic circuits interconnecting each other electrically or mechanically, or subsystems such as a number of extremely small passive components and metalization patterns combined in an arrangement. Such passive components and metalization patterns can be physically isolated and buried adjacent to one another in a single interconnect circuit board to electrically connect to each other and/or extend from the interconnect circuit board thereby. Nowadays, ceramic compositions are usually applied for the interconnect circuit board.
In the ceramic composition, complex electronic circuits generally require several insulating dielectric layers to separate layers of conductors. Meeting the requirement of different dielectric constants (K) suitable for fabricating or burying passive components and metalization patterns, a series of dielectric material with different dielectric constants are needed. For example, a low dielectric constant material is preferred in a signal processing portion of the ceramic composition to improve the speed at which the signals therein are propagated to provide fast processing; and a high dielectric constant material is preferred in fabricating capacitors as a buried passive component. The electrically conductive pathways interconnecting the passive components and metalization patterns through the dielectric layers are called vias. Such a multiplayer structure allows a circuit to be more compact and occupy less space.
The method for co-firing a multiplayer ceramic composition on which passive components and metalization patterns such as resistors, capacitors, or conductors have been printed with metallized vias extending through the dielectric layers to interconnect various passive components and metalization patterns, is described in U.S. Pat. No. 4,654,095 incorporated herein for reference. The ceramic powder can densify at temperatures that are compatible with the use of high conductivity metallizations such as gold, silver, and copper. In particular, densification is achieved at or below 1000° C. in order to provide a margin of error sufficiently distant from the melting point of gold, 1060° C. The dielectric layers are stacked in registry and pressed together at a proper temperature and pressure, and then fired to drive off the organics such as binders and plasticizers in the green ceramic body. All the ceramic and heterogeneous materials are sintered and densified thereby. This method has the advantages of performing firing only once, saving fabricating time and labor, and limiting the diffusion of mobile metals to prevent shorting between the conductive layers.
However, co-firing a monolithic structure having a high K dielectric and a low K dielectric material has problems. One of the problems is the changes of electric properties and the other is shrinking mismatch occurring during firing.
In the respect of the changes of electric properties, many conventional assembles utilize a low dielectric constant material and a high dielectric constant material, the low dielectric constant material containing a glass resulting in increased dielectric constant and increased losses, and the high dielectric constant material containing lead, magnesium and niobium. However, chemical reactions take place when a low dielectric material and a high dielectric material are in contact at temperatures above 800° C. during the co-firing process due to interface diffusion. For this reason, the dielectric constants of both low and high dielectric constant materials are both changed. Usually, a dramatic drop of the high dielectric constant material happens.
Several buffer layers interposed between a low and a high dielectric constant materials of a electronic package are disclosed in U.S. Pat. No. 5,757,611. The buffer layers containing 25 to 100% barium compound provide an additional physical barrier during the initial stages of densification by creating a more difficult path for chemical diffusion. Vias can also be formed through the buffer layers for electrical conducting a passive component portion and a signal processing portion. The glass forming additives and inorganic fillers controls the shrinkage rate, thermal expansion and chemical compatibility of the buffer layer in contact with either the high K dielectric layers of the passive component portion or the is low K dielectric layers of the signal processing portion. On the other hand, the buffer layer enlarges the thickness of the electronic package, and thus cannot serve as a good substrate for burying passive components, either.
Other multilayer ceramic green tape structure with different K is shown in U.S. Pat. No. 6,055,151, which focuses on inks screen printed onto low firing temperature green tapes to form embedded components such as capacitors to tight tolerances with high precision placement. The capacitor layer is sandwiched between two barium titanate barrier layers having a thickness sufficient to prevent diffusion of the low firing temperature glass of the green tapes. Furthermore, the multilayer ceramic green tape structure is bonded on a metal support substrate by bonding glass to prevent shrinkage. However, the shrinkage rates of the low firing temperature green tapes and the metal support substrate are different, and should be well controlled during firing to avoid fracture occurring. Besides, the thickness of the multilayer ceramic green tape structure still cannot be reduced.
In the respect of the shrinkage, because the shrinkages of elements when sintering are not the same, the firing condition is difficult to control. Furthermore, this uncertainty in the X and Y dimensions resulting in misregistration during assembly of large and complex circuits is particularly undesirable. A method for reducing shrinkage during firing of green ceramic bodies is disclosed in U.S. Pat. No. 5,085,720. Each of the top and bottom of the green ceramic body is applied with a release layer to form a “sandwich” structure. During burning out and sintering, a unidirectional pressure is applied to the surface of the release layer. The porosity of the release layer provides an escape pathway for the volatile components of the green ceramic body. Because the release layer does not shrink in firing, the shrinkage in the X and Y dimensions of the green ceramic body is reduced. On the other hand, the release layers covering both the top and bottom surfaces of the green ceramic body, their removal should be performed after sintering for printing with and firing the conductors, resistors, and capacitors thereon. The cost of the method thus rises. When fabricating a large number of ceramic layers (e.g. more than 6 layers), middle layers of a green ceramic body still shrink as the result of the forces not being distributed evenly by applying the release layers on the top and bottom of the green body (i.e. the forces on the top and the bottom and on the middle layers of the green body are substantially different).
Some modifications of the green ceramic bodies disclosed in U.S. Pat. No. 5,085,720 had been made, in which a constraining layer for preventing shrinkage is set between the layers of a green ceramic body. The constraining layer remains in the final product preventing the disadvantage of removal. However, the constraining layer cannot be as a proper dielectric material; the thickness of the product is therefore enlarged.
In order to resolve the problems mentioned above, the present invention develops a novel multilayer ceramic composition that has the advantages of downsizing and a better circuit precision by reducing shrinkage in the X and Y dimensions when co-firing two dielectric materials with different dielectric constants and passive components buried therein.